Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report the achievement of transport critical currents in Sr_0.6K_0.4Fe₂As₂ wires and tapes with a T_c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J_c was enhanced upon silver addition, and at 4.2 K, a largest J_c of ～1200 A/cm² (I_c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J_c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J_c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.     

Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method

The recent discovery of superconductivity in the iron-based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high-T_c iron pnictide composite wires, such as LaFeAsO_1?xF_x, SmFeAsO_1?xF_x and Sr_1?xK_xFeAs, has been fabricated by the in situ PIT technique using Fe, Ta and Nb tubes. Microscopy and X-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. Furthermore, the wires exhibit a very weak J_c-field dependence behavior even at high temperatures. The upper critical field H_c2(0) value can exceed 100 T, surpassing those of MgB₂ and all the low temperature superconductors and indicating a strong potential for applications requiring very high field. These results demonstrate the feasibility of producing superconducting pnictide composite wire. We also applied the one-step PIT method to synthesize the iron-based bulks, due to its convenience and safety. In fact, by using this technique, we have successfully discovered superconductivity at 35 K and 15 K in Eu_0.7Na_0.3Fe₂As₂ and SmCoFeAsO compounds, respectively. These clearly suggest that the one-step PIT technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of novel iron-based superconductors.     

Transition temperature model based on specific heat data of an FeAs-based superconductor

In this article, we studied the specific heat data of iron-based superconductors LaO_{1? x} F _x FeAs (x?=?0.1) and SmO_{1? x} F _x FeAs (x?=?0.13,?0.12, and 0.1). (i) The contribution of phonons in specific heat above T_c depends exponentially on temperature. (ii) The specific heat has different contributions, and they change differently at T_c . This change must be the effect of a physical function on heat capacity. Therefore, transition temperature is defined by thermal parameters. For LaO_0.9F_0.1FeAs and SmO_0.87F_0.13FeAs, a transition point was evaluated at 22.11 and 26.32?K, respectively. This is in close agreement with the midpoint transition temperatures obtained from dc resistivity and magnetization experiments, where the modified electronic heat capacity led to the approximate value of the transition point in two samples. The jump, ΔC/γT _c, and the electronic and lattice heat capacity coefficients, γ and β, respectively, were evaluated.     

Quantum critical point in SmO1−xFxFeAs and oxygen vacancy induced by high fluorine dopant

The local lattice and electronic structure of the high‐T_c superconductor SmO_1–xF_xFeAs as a function of F‐doping have been investigated by Sm L₃‐edge X‐ray absorption near‐edge structure and multiple‐scattering calculations. Experiments performed at the L₃‐edge show that the white line (WL) is very sensitive to F‐doping. In the under‐doped region (x≤ 0.12) the WL intensity increases with doping and then it suddenly starts decreasing at x = 0.15. Meanwhile, the trend of the WL linewidth versus F‐doping levels is just contrary to that of the intensity. The phenomenon is almost coincident with the quantum critical point occurring in SmO_1–xF_xFeAs at x? 0.14. In the under‐doped region the increase of the intensity is related to the localization of Sm‐5d states, while theoretical calculations show that both the decreasing intensity and the consequent broadening of linewidth at high F‐doping are associated with the content and distribution of oxygen vacancies.     

Very high upper critical fields of F-doped Fe-based layered superconductors NdO0.88F0.12FeAs and CeO0.88F0.12FeAs

Using the solid state reaction method, we have synthesized the polycrystalline F-doped NdO_0.88F_0.12FeAs and CeO_0.88F_0.12FeAs with the superconducting transition temperatures at about 48 and 40 K, respectively. To obtain the upper critical field H _c2 of Nd(Ce)O_0.88F_0.12FeAs samples, we measured the electrical resistivity under magnetic field up to 14 T. Based on the Werthamer-Helfand-Hohenberg (WHH) relation together with the H _c2(T) curves in a relatively high field, we estimated that these superconductors have a rather high upper critical field of about 115 T for Nd-based and 107 T for Ce-based samples, indicating the similarities between these ReO_1−x F _x FeAs (Re = rare earth element) superconductors and high T _c cuprate superconductors. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Basic Research Program of China (973 Program)(Grant No. 2006CB9213001) and the National Natural Science Foundation of China (Grant No. 10774181)     

Role of chemical pressure in enhancing the transition temperature (T<Subscript>c</Subscript>) and upper critical field (H<Subscript>c2</Subscript>) in the Y-doped Ce-oxyfluoride superconductor

Structural and superconducting properties of yttrium substituted Ce_1-xY_x(O/F)FeAs superconductors have been investigated for the first time. All the compounds crystallize in the tetragonal ZrCuSiAs structure type. There is a decrease in both the a and c lattice parameters on increasing yttrium substitution (with fixed F content) along with a substantial enhancement of the superconducting transition temperature (T_c) and upper critical field (H_c2) indicating the influence of chemical pressure. Interestingly the maximum T_c (~48 K) was observed for an intermediate composition (Ce_0.5Y_0.5O_0.9F_0.1FeAs) which is higher than either of the parent Y or Ce-compounds, (YO_0.9F_0.1FeAs (~10 K) and Ce(O/F)FeAs (~42 K)). The transition temperature was also found to be nearly independent of the electron -doping introduced by fluoride substitution (0.1 to 0.2 moles per formula unit) indicating the significance of the charge reservoir layer (Ce-O). The yttrium substituted (fluoride free) compositions of the type, Ce_1-xY_xOFeAs were found to be semimetallic like the parent compound CeOFeAs with the shift in the anomaly temperature towards low temperature on substitution of yttrium ions. Hall coefficient and thermopower measurements show an increase in charge carriers (electrons) through Y-doping in fluorine doped CeOFeAs.     

Improved critical current capability of V3Ga formed in a V-6Ga/Cu-15 Ga composite

Single filament V₃Ga composite wires were prepared by imbedding cores of pure V and V-6.1 at.% Ga in a Cu-15.4 at.% Ga matrix. The parabolic growth rate of V₃Ga was enhanced by alloying the core, and the superconducting critical current density was nearly doubled. At 4.2 K J_c exceeded 10⁵ A/cm² in transverse magnetic fields up to 140 kG.     

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

Influences of Cu composition and sintering condition in Bi-2223 tapes using Ag---Cu alloy sheath doped with Ti, Zr or Hf

Bi-2223 Ag---Cu alloy sheathed tapes doped with Ti, Zr or Hf were fabricated by a powder-in-tube technique using intermediate uniaxial presses and variations in total sintering times. Increasing Cu compositions in the Ag---Cu alloy sheath doped with certain elements into the sheath, combined with appropriate total sintering times were found to lead good superconducting properties. Furthermore, the best total sintering time varied according to the amount of Cu in the Ag---Cu alloy sheath and the existence of doping elements. A transport critical current density, J_c, of 4.1 × 10⁴ A/cm² at 4.2 K, 14 T and 2.2 × 10⁴ A/cm² at 77 K, 0 T was achieved in Ag-10 at% Cu-0.1 at% Hf sheathed Bi-2223 tapes sintered for total 200 h.     

New superconductor with a layered crystal structure: Nickel oxybismuthide LaO<Subscript>1−δ</Subscript>NiBi

The results of synthesizing a new layered phase—nickel oxybismuthide LaO_1?δNiBi in a series of superconducting oxypnictides—and its properties in the superconducting and normal states are reported. Although the temperature of the transition of this phase to the superconducting state, T _c ～ 4 K, is much lower than the value T _c = 55 K reached at present in oxyarsenide SmO_1?δFeAs, the similarity of the crystal structures and ρ(T) dependencies indicates that the mechanism responsible for the appearance of the semiconducting state is the same in lanthane oxybismuthide and samarium oxyarsenide.     

Charge redistribution and a shortening of the Fe—As bond at the quantum critical point of SmO1–xFxFeAs

Many researchers have pointed out that there is a quantum critical point (QCP) in the F‐doped SmOFeAs system. In this paper, the electronic structure and local structure of the superconductive FeAs layer in SmO_1–xF_xFeAs as a function of the F‐doping concentration have been investigated using Fe and As K‐edge X‐ray absorption spectroscopy. Experiments performed on the X‐ray absorption near‐edge structure showed that in the vicinity of the QCP the intensity of the pre‐edge feature at the Fe‐edge decreases continuously, while there is a striking rise of the shoulder‐peak at the As edge, suggesting the occurrence of charge redistribution near the QCP. Further analysis on the As K‐edge extended X‐ray absorption fine structure demonstrated that the charge redistribution originates mostly from a shortening of the Fe—As bond at the QCP. An evident relationship between the mysterious QCP and the fundamental Fe—As bond was established, providing new insights on the interplay between QCP, charge dynamics and the local structural Fe—As bond in Fe‐based superconductors.     

Superconductivity at 36 K in gadolinium-arsenide oxides GdO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeAs

In this paper we report the fabrication and superconducting properties of GdO_1−x F _x FeAs. It was found that when x is equal to 0.17, GdO_0.83F_0.17FeAs is a superconductor with the onset transition temperature T _c^on ≈ 36.6 K. Resistivity anomaly near 130 K was observed for all samples up to x = 0.17, and such a phenomenon is similar to that of LaO_1−x F _x FeAs. Hall coefficient indicates that GdO_1−x F _x FeAs is conducted by electron-like charge carriers. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Natural Science Foundation of China (Grant Nos. 10221002/A0402 and 10774170/A0402), the National Basic Research Program of China (973 Program) (Grant Nos. 2006CB601000, 2006CB921107 and 2006CB921802), and the Chinese Academy of Sciences (ITSNEM)     

High-temperature superconductivity in transition metal oxypnictides: A rare-earth puzzle?

Extensive ab initio LDA and LSDA + U calculations of an electronic structure of newly discovered high-temperature superconducting series ReO_{1 − x} F _x FeAs (Re = La, Ce, Pr, Nd, and Sm and the hypothetical case of Re = Y) have been performed. In all cases, almost identical electronic spectrum (both energy dispersions and the densities of states) has been obtained in a rather wide energy interval (about 2 eV) around the Fermi level. This fact is unlikely to be changed by strong correlations. This leads inevitably to the same critical temperature T _c of a superconducting transition in any theoretical BCS-like mechanism of the Cooper pair formation. The experimentally observed variations of the T _c for different rare-earth substitutions are either due to the disorder effects or less probably because of possible changes in the spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers. The text was submitted by the authors in English.     

利用电泳法在金属基底上制备MgB2超导厚膜

利用电泳技术在高熔点金属基底Ta，Mo和W上制备MgB₂超导厚膜.厚膜中的MgB2晶粒结合紧密，粒度小于1μm，呈随机取向生长.电阻测量表明沉积在Ta，Mo，W上的MgB₂厚膜的超导起始转变温度分别为36.5K，34.8K，33.4K，对应的转变宽度为0.3K，1.5K和2.0K.三种基底上制备的MgB₂厚膜的临界电流密度在不同温度下随外磁场的变化情况 基本相同，MgB₂/Mo厚膜的临界电流密 关键词： 2超导厚膜')" href="#">MgB₂超导厚膜 电泳 金属基底     

Angle Integrated Photoemission Study of SmO0.85F0.15FeAs

The electronic structure of the new superconductor SmO_1-xF_xFeAs (x=0.15) is studied by angle-integrated photoemission spectroscopy. Our data show a sharp feature very close to the Fermi energy, and a relative flat distribution of the density of states between 0.5eV and 3eV binding energy, which agrees well with the band structure calculations considering an antiferromagnetic ground state. No noticeable gap opening is observed at 12K below thesuperconducting transition temperature, indicating the existence of large ungapped regions in the Brillouin zone.     

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO_0.88F_0.12FeAs samples with bulk critical temperature T _c = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are Δ _L = 10.5 ± 2 meV and Δ _s = 2.3 ± 0.4 meV, respectively.     

122型铁基超导线带材实用化研究进展

在种类众多的新型铁基超导材料中,122型铁基超导体具有高转变温度、超高上临界场、低各向异性、高临界电流密度等优点,因此成为高场应用领域最具竞争力的铁基超导材料.目前122型铁基超导线带材在4.2 K,10 T下的传输临界电流密度已经超过10⁵A/cm²这一实用化门槛值,表现出十分广阔的应用前景.本文回顾了新型铁基超导体的发现及发展历程,结合122型铁基超导体的自身特点,就如何制备高性能122型铁基超导线带材展开讨论,同时对粉末装管法制备流程中影响线带材性能的几大关键因素进行了详细分析.重点介绍了近年来122型铁基超导线带材的实用化研究进展,包括高强度线带材的制备、圆线的研制、多芯线材及长线的制备、超导接头的研究、力学性能及各向异性的研究等.对122型铁基超导线带材实用化研究进行了总结,并对其未来的发展趋势进行了展望.     

Anionic substitutions in Bi-2223 superconductor

Samples of nominal stoichiometry Bi_1.6Pb_0.4Sr₂Ca_2.5Cu_3.5O_x−zF_z and Bi_1.6Pb_0.4Sr₂Ca_2.5Cu_3.5O_x−z(SO₄)_z (z=0, 0.3, 0.6 and 0.9) were prepared by standard solid state reaction from appropriate oxides, carbonates, copper (II) fluoride, and copper (II) sulphate. Superconducting properties of the samples were characterized by measuring of temperature dependency of their electrical resistivity and critical current density, their phase composition was determined by X-ray diffraction (XRD) and element composition by X-ray fluorescence (XRF). While greater amount of fluorine widens the superconducting transition and worsens transport properties, the less doped sample shows increase in critical current density. Sulphur-doped samples show no superconducting transition at all.     

Some superconducting properties of 25%Nb–75%Zr alloy

The manner of preparation of superconducting 25% wt Nb–75% wt Zr wires is described. Short samples of these wires were measured in static magnetic fields up to 80·5 kG and the authors describe the method of these measurements. The paper gives the results of measuring the critical current density dependence on the external perpendicular magnetic field for both cold-worked wires with different deformations and heat-treated wires. The dependence of the critical current density on the heat treatment temperature after wire deformation for different magnetic fields was obtained. The optimum heat treatment temperature (vacuum better than 10^–3 torr, 1 hour) is 450–600C for magnetic fields 0–80·5 kG. The values ofi _c of these wires in magnetic fields up to 60 kG are the same or higher than those of 75% Nb-25% Zr wires, and in fields above 60 kG they are much higher.     

Determination of foreign phases in Fe–As based superconducting systems

The recently discovered superconducting materials, containing Fe and As, have raised huge interest. However most materials prepared to date, suffer from a varying degree of content of foreign phases, Fe₂As, FeAs₂ and FeAs, which can lead to wrong conclusions concerning the properties of these materials. Mössbauer Spectroscopy determines quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of eight samples of three Fe–As based families of materials, RAsFeO_1???x F _x , Sr_1???x K _x Fe₂As₂ and LiFeAs, prepared in three different laboratories.